Check valves are in widespread use, serving to prevent flow in a reverse direction along a passage. A typical check valve comprises a valve member which is urged against an associated seat by a spring or other resilient biasing load, the valve member being able to be lifted from the seat against the action of the biasing load by the fluid pressure upstream of the check valve when that pressure exceeds that downstream of the check valve by a predetermined level. If the pressure difference is lower than the aforementioned predetermined level, or indeed if the pressure downstream of the valve is greater than that upstream of the valve as would be the case if fluid were flowing in the reverse direction, the biasing load serves to firmly seat the valve member against the seat, preventing fluid flow through the valve.
There is the risk that, for example, damage to the valve member or damage to the seat may result in the valve being incapable of fully sealing even when the valve member is engaged against the seat. Accordingly, fluid may be able to leak past the valve even when the valve is closed. This is undesirable.
Likewise, where the valve member has a conical shaped sealing surface, and valve seat takes a corresponding conical form, there is a risk that slight tilting or misalignment of the valve member with the seat can result in ineffective sealing thereof, and so can result in leakage.
It is known to provide such a valve with an environmental seal to improve leakage resistance. By way of example, a flexible seal member may be attached to the valve member and engageable with the seat. However, there is a tendency, over time, for the flexible seal member to deform or extrude, resulting in ‘tails’ of the flexible seal member material forming which may become trapped between the valve member and the seat, causing or increasing leakage through the valve.